Ballast circuits for discharge lamps

ABSTRACT

A ballast circuit for a discharge lamp is arranged to supply a uni-directional voltage to the lamp which is made up of at least two current components which differ in phase. Flicker at the supply frequency f and at 2f is thus much reduced. A preferred ballast for use with a three-phase supply employs ballast capacitors in each of the three supply lines connected to a three-phase bridge rectifier, the output of which is applied to the lamp. A preferred ballast for use with a single phase supply includes a ballast capacitor and a ballast inductor connected through respective bridge rectifiers which have some components in common.

United States Patent Farrow Jan. 22, 1974 [54] BALLAST CIRCUITS FORDISCHARGE 3,249,809 5/1966 Steiner! 315/010. 5 3,676,735 7/1972 Roche315/207 LAMPS [75] Inventor: gictfr grancis Farrow, London, PrimaryExaminer Herman Karl Saalbach ng an Assistant ExaminerJames B. Mullins[73] Assignee: Thorn Electrical Industries Limited, Attorney, Agent, orF inn-Robert OCOImeII;

London, England Dike, Bronstein, Roberts & Cushman PP No.3 279,445 Aballast circuit for a discharge lamp is arranged to supply auni-directional voltage to the lamp which is 52 us. (:1 315/137,315/205, 315/207, made "P of at least two Current cmnponems which315/227 R, 315/244, 315/283, 315/D1G. 5 fer in phase. Flicker at thesupply frequency fand at 51 Int. Cl. H05b 41/16 f is thus much APreferred ballast for use 58 Field 61 Search... 315/137, 139, 141, 205,207, w a three-Phase pp y p y ballast capacitors 315/244" 7 R, 7 283,DIG 5 In each of the three supply lines connected to a threephase bridgerectifier, the output of which is applied 5 References Cited to thelamp. A preferred ballast for use with a single UNITED STATES PATENTSphase supply includes a ballast capacitor and a ballast 5 inductorconnected through respective bridge rectifiggY/gggg 315/l37 ers whichhave some components in common- 3,054,884 9/1962 Manz et al. 315/207 X 2Claims, 8 Drawing Figures 5 F US E I I O O I F E 3 1 PHASE I I A. C. F1.

14 1 BRIDGE 10 12 RECTIFIER l l IF PATENTEU SHEET 1 (1F 4 FUSES BRIDGERECTIFIER 1IIL 14 F/G. 7f

LA MP5 LINE CURRENT SHEET 2 OF 4 ounykwmmvr A1 V V 3 PHASE A.C. Q R Q Q3 PHASE 3 M INDUCTOR 7 *K30 [D LAMP L BRIDGE RECTIFIER PATENTEU P in 3,787 751 SHEET 3 OF 4 SINGLE PHASE AC.

1 BALLAST CIRCUITS FOR DISCHARGE LAMPS BACKGROUND OF THE INVENTIONThisinvention relates to ballast circuits for discharge lamps.

Gas discharge lamps are conventionally operated from alternating currentsources by using a ballast circuit. A basic type of ballast circuitincludes an inductor in series with the lamp to absorb the differencebetween the supply voltage and the lamp voltage; because the inductor isreactive it consumes a relatively. small, though significant, amount ofpower. A capacitor may be connected across the series combination of theinductor and lamp to correct the power factor presented to the supply.

Modifications of this basic circuit include the use of auto-transformersand auto-transformers with built-in leakage reactance. However,capacitors have not been used in place of the inductor because they giverise to undesirable current waveforms, the rapid charging anddischarging being sufficient to damage the lamp and also produce markedflicker. Thus, where a leading power factor is required inductors andcapacitors are used in series, with the capacitive reactance beinggreater than the inductive reactance. The inductive reactance has theeffect of improving the lamp current waveform while the overall ballasteffect is capacitive.

All these ballasts produce flicker in the light output at the mainssupply frequency f and at 2f. The psychological implications of thisflicker are not fully understood, but it is known that the flicker cancause some considerable distress.

We have appreciated that one way of eliminating flicker is to operatethe lamp on a direct current supply. We have therefore been concerned toprovide circuits which will enable a lamp to be run on an effectivelydirect current from an alternating current source, while still providinga ballast effect and not being unnecessarily complex, and in such a waythat unacceptable flicker is at least substantially reduced.

SUMMARY OF THE INVENTION According to this invention there is provided aballast circuit for a discharge lamp, comprising input terminals forreceiving analternating voltage, output terminals for applying auni-directional voltage to a discharge lamp, and means connected betweenthe input and output terminals for applying to the output terminals atleast two current components which differ in phase.

Another disadvantage of known ballasts is that they require a bulkyiron-cored inductor, which is expensive and which results in an overallefficiency of only about 90 percent, measured as a percentage of thepower input from the source which actually reaches the lamp. An addedadvantage of one ballast circuit embodying this invention is that itdoes not require the use of an inductor.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described byway of example and with reference to the accompanying drawings, inwhich: I

FIG. 1 is a circuit diagram of a ballast circuit for operating adischarge lamp on direct current from a threephase alternating currentsource, the ballast circuit not including any inductors;

FIG. 2 is a waveform diagram showing the line current and phase voltagefor one phase of the supply in FIG. 1;

FIG. 3 is a waveform diagram showing the output current from the ballastcircuit of FIG. 1;

FIG. 4 is a circuit diagram of another ballast circuit for operating adischarge lamp on direct current from a three-phase alternating currentsource;

FIG. 5 is a circuit diagram of a ballast circuit for operating adischarge lamp on direct current from a single-phase alternating currentsource;

FIG. 6 is a waveform diagram showing the input current to the ballastcircuit of FIG. 5;

FIG. 7 is a waveform diagram showing the output current from the ballastcircuit of FIG. 5; and

FIG. 8 is a waveform diagram showing certain voltages in the circuit ofFIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS limiting resistor 24, acrossthe terminals 20.

The alternating currents through each of the capacitors 14 are identicalin form but phase displaced relative to each other by 120. The linecurrent relative to phase voltage for one capacitor is shown in FIG. 2.The output current from the bridge rectifier 16 is shown in FIG. 3 andis seen to consist of a uni-directional current with a ripple thefrequency of which is six times the supply frequency. The rippleamplitude is relatively small, the ratio A:B being typically about 1:3.

At this ripple frequency the afterglow of the lamp phosphors issignificant, and this gives a further reduction in light output ripple.Thus this ballast circuit provides a very substantial reduction inripple without the use of a complex ballast circuit. Surprisingly, it isfound to operate well while using capacitors alone without inductors,and the efficiency is thereby increased to upwards of percent. Theballast circuit provides a leading power factor which can be used tohelp correct a lagging load in factory installations.

If the supply voltage is 415 volts,the prestrike voltage generated.namely 415 X\/ 2 volts, is sufficient to strike two normal 400 watt highpressure mercury vapour lamps in series.

FIG. 4 shows a similarballast circuit in which the three capacitors 14are replaced by a three-phase inductor 30. In this case the reactancepresented to the supply is inductive, and the circuit will provide alagging power factor which can be used in conjunction with the capacitorballast circuit of FIG. 1 to at least partially correct the powerfactor. The three-phase inductor 30 could be replaced by threeindividual inductors connected in the three lines. In other respects theballast circuit of FIG. 4 is similar to that of FIG. 1, and includes abridge rectifier the output of which is connected to a discharge lamp.

Another ballast circuit is shown in FIG. 5. Two input terminals 40 areconnected to a discharge lamp 42 through a double bridge circuit 44. Thecircuit 44 may be considered as two superimposed bridge circuits, withhalf of each bridge being common. The common elements are diodes 46 and48 which are connected in opposite senses between one of the terminals40 and respective output terminals 41 which are connected to the lamp42. To the other terminal 40 are connected both a capacitor 49 and aninductor 50. Four further diodes 52, 54, 56 and 58 are then connected asfollows: diode 52 between one end of the lamp 42 and the capacitor 49,diode 54 between the same end of the lamp and the inductor 50, diode 56between the other end of the lamp and the capacitor, and diode 58between the said other end of the lamp and the inductor. The cathode of.diodes 46, 52 and 54 are all connected together and to one end of thelamp, and the anodes of diodes 48, 56 and 58 are all connected togetherand to the other end of the lamp.

Thus the lamp is supplied through both the capacitor 49 and the inductor50 with current which leads and lags the voltage respectively, and thelamp 42 thus receives two current components of differing phase. Theeffect is to produce an input line current as shown in FIG. 6, and anoutput current as shown in FIG. 7, in which alternate peaks correspondto component currents through the capacitor 49 and inductor 50respectively. The fundamental of the line current is in phase with thesupply voltage. It will be seen that the output current includes a larged.c. component and a ripple of predominantly four times the supplyfrequency.

To reduce the twice-line-frequency ripple component to a minimum, thecapacitor reactance /21fC) should be approximately equal to the inductorreactance (2 rrfL). This results in a larger capacitor than is normallyused for power factor correction.

The open-circuit prestrike voltage V across the lamp is seen to consistof the sum of the open-circuit voltages V and V across the diodes 46 and48 respectively. The forms of the voltages V V and V are shown in FIG.8. The peak open-circuit prestrike voltage (V produced by the circuit ofFIG. 5 with a 240 volts r.m.s. input is 240' 2 volts, which issufficient to strike a high pressure mercury vapour discharge lamp.

I claim:

1. A ballast circuit for a discharge lamp, comprising:

first and second input terminals for receiving an alternating voltage;

first and second output terminals for applying a unidirectional voltageto a discharge lamp;

two oppositely-poled diodes connected between said first input terminaland said first and second output terminals respectively;

a capacitor and an inductor both connected to said second inputterminal;

two oppositely-poled diodes connected between said capacitor and saidfirst and second output terminals respectively; and

two oppositely-poled diodes connected between said inductor and saidfirst and second output terminals respectively.

2. In combination with a discharge lamp, a ballast circuit comprising:

three input terminals for connection to a three-phase alternatingcurrent supply;

two output terminals for applying a uni-directional voltage to saiddischarge lamp;

three ballast capacitors each connected to a respective input terminal;and

a three-phase bridge rectifier connected between said capacitors andsaid output terminals, whereby said voltage applied to said dischargelamp has a ripple frequency which is six times the frequency of thealternating current received by said input terminals.

1. A ballast circuit for a discharge lamp, comprising: first and secondinput terminals for receiving an alternating voltage; first and secondoutput terminals for applying a uni-directional voltage to a dischargelamp; two oppositely-poled diodes connected between said first inputterminal and said first and second output terminals respectively; acapacitor and an inductor both connected to said second input terminal;two oppositely-poled diodes connected between said capacitor and saidfirst and second output terminals respectively; and two oppositely-poleddiodes connected between said inductor and said first and second outputterminals respectively.
 2. In combination with a discharge lamp, aballast circuit comprising: three input terminals for connection to athree-phase alternating current supply; two output terminals forapplying a uni-directional voltage to said discharge lamp; three ballastcapacitors each connected to a respective input terminal; and athree-phase bridge rectifier connected between said capacitors and saidoutput terminals, whereby said voltage applied to said discharge lamphas a ripple frequency which is six times the frequency of thealternating current received by said input terminals.